1. Field of the Invention
The present invention relates to a device for drying a web in a two-screen dryer section, e.g., in a paper manufacturing machine, and for discharging paper shreds and foreign objects entering a loop or shaft formed by the dryer screen.
2. Discussion of Background Information
In the prior art, several devices, e.g., U.S. Pat. No. 5,379,526 and German Patent Application No. DE 44 04 726, have been proposed for removing paper and foreign objects which fall through a dryer screen.
The prior art documents discuss, in general, that upon leaving a dryer cylinder of a first cylinder row, the web to be dried is guided by the dryer screen until the web is lifted off at the screen guide roll. The web then runs, via a free draw in which the web is not supported, to a cylinder of the second cylinder row. This arrangement prevents the risk web flutter and web breaks. Thus, in regions in which the web and/or dryer screens are not running along a defined surface, such as a cylinder surface or guide roll surface, the web and dryer screens are able to steam out without interference.
The dryer screen, after separating from the web, is guided around the screen guide roll to an adjacent dryer cylinder in the cylinder row. The screen forms a loop or shaft and an air box is located within the loop or shaft. A sealing element, supported by the air box, prevents the air boundary layer from being pulled into the shaft and feeds the air boundary layer through the screen running around the first cylinder.
In a region in which the dryer screen loop is not covered with paper, a second air flow exists. The air flow is required to ventilate the "pocket," i.e., the region between the dryer screen and a portion of the web being guided onto the adjacent cylinder in the same row, through the dryer screen. The pocket ventilation is provided in DE '726 via a blowing device positioned in a "discharge wedge" between the screen guide roll and the discharge screen. The blowing device consists of a driving nozzle creating a vacuum to apply suction to the web leaving the first cylinder.
However, the above-discussed methods of vacuum generation, i.e., in a first section of the shaft, and/or pocket ventilation have been shown to be inadequate. In recent years, the operating speed of modern paper manufacturing machines has increased significantly, i.e., in an order of magnitude of 1800 m/min and greater. Thus, an increased drying capacity per dryer cylinder and an increase in the dry air volume is required to remove steam. Therefore, more air must be pushed through the dryer screen into the pocket. Further, in spite of increased speed, the web runs must be maintained as quietly as possible.
For this purpose, German Patent Application No. 195 27 289.7 discloses that, in keeping with the requirements of the system, an air flow at any pressure is fed separately to the blowing device so as to intensify the pocket ventilation, and that an independent second air flow is provided to enable the adjustment of the vacuum present in the shaft via a driving jet. For this purpose an air duct is provided between the air box and the blowing device, which starts out from a first part of the shaft. This advantageously provides a blowing device pressurized with any rate of air flow to randomly intensify the pocket ventilation. By subdividing the air supply system into two subsystems, partial functions can be affected individually.
However, a disadvantage of the systems of DE '726 and DE '289 is that, in the region between the blowing device and the dryer screen discharged by the guide roll, there is a space limited by longitudinal seals in which paper shreds occasionally accumulate which are difficult to remove. Further, in the event of a web break, the paper shreds arrive at a "wrong" side of the dryer screen and are transported by the screen into the shaft and may be caught in the space.